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Platinum distribution and electrocatalytic properties of modified polypyrrole films
Electrochimica Acta 46 (2000) 661– 670 www.elsevier.nl/locate/electacta
Platinum distribution and electrocatalytic properties of modified polypyrrole films K. Bouzek 1,2, K.-M. Mangold, K. Ju¨ttner *,1 Karl-Winnacker-Institut der DECHEMA e.V., Theodor-Heuss-Allee 25, D-60486 Frankfurt am Main, Germany Received 12 April 2000; received in revised form 18 August 2000
Abstract For the preparation and modification of conducting polymer films as electrocatalysts for anodic hydrogen oxidation three pathways were investigated: (i) cathodic deposition of platinum from a hexachloro-platinum complex [PtCl6]2 − on the pre-synthesised polymer film, (ii) incorporation of colloidal platinum particles into the polymer film during electropolymerisation and (iii) incorporation of a tetrachloro-platinum complex [PtCl4]2 − during the electropolymerisation as counter ion and its subsequent cathodic reduction. Using SEM and EDX the distribution of platinum in the polypyrrole film depending on the preparation conditions was studied. The catalytic activity of the modified films was derived from polarisation curves of hydrogen oxidation using rotating disk electrode technique. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: Polypyrrole; Platinum; Distribution; Electrocatalysis; Hydrogen oxidation
1. Introduction Conducting polymers have been studied intensively in recent years because of their possible utilisation in many practical applications, such as charge storage devices, electrochromic displays, modified electrodes, anti-corrosion coatings, molecular transistors and high surface area support for catalyst microparticles [1]. The incorporation of catalyst microparticles into a conducting polymer film represents an attractive field of research. The catalytic activity of platinum incorporated into a conducting polymer layer for the oxidation of hydrogen or for small organic molecules has been the subject of previous investigations [2–13]. Polypyrrole and polyaniline are the most common con* Corresponding author. Fax: +49-6975-64388. 1 ISE member. 2 On leave from the Department of Inorganic Technology, Institute of Chemical Technology, Technicka´, 166 28 Prague 6, Czech Republic.
ducting polymers in these studies. Several ways of preparing platinum modified conducting polymer films have been reported in the literature. They can be divided into three main groups: (i) electrochemical Pt deposition onto the previously synthesised polymer film [4 – 18], (ii) incorporation of colloidal Pt particles prepared by chemical reduction of hexachloroplatinate during electropolymerisation of the film [2,19] and (iii) incorporation of tetrachloroplatinate as a counter ion and its subsequent cathodic reduction after electropolymerisation of the polymer film [13– 17,19,20]. Most authors suppose the platinum microparticles to be homogeneously dispersed over the conducting polymer film [4,7,8,10,11,13– 17,19,20]. In a few papers a two-dimensional distribution is indicated if the platinum particles were cathodically deposited on the presynthesised polymer film [13,16,21]. It is the aim of this work to compare the above-mentioned methods with respect to the distribution of platinum in the polymer film and the catalytic activity of the composite layer for the hydrogen oxidation reaction.
0013-4686/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 0 0 ) 0 0 6 5 9 - 9
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2. Experimental Electrochemical measurements were performed with a HEKA Potentiostat-Galvanostat (PG310) using a three-electrode configuration in a double compartment glass cell. All potentials refer to the SCE if not otherwise indicated. The nozzle of the Luggin capillary is filled with a magnesium rod in order to minimise the diffusion of Cl− ions into the electrolyte solution and to avoid poisoning of the Pt modified electrode. The chemicals were commercial materials of the highest available purity and were used without further purification with the exception of pyrrole Py, which was purified by distillation. In all cases the polypyrrole films (PPy) were synthesised potentiostatically at 0.75 V, this is close to the potential used by Bose and Rajeshwar [19] and reported to correspond to approximately 90% efficiency of PPy electropolymerisation. The temperature was kept constant at 20°C, the thickness of the films was controlled by the charge flowing during the anodic electropolymerisation. A charge of 100 mC cm − 2 was assumed to correspond to a film thickness of 0.25 mm [10,22]. Compared with previous investigations, relatively thick films of typically 9 91 mm were synthesised to permit a more detailed study of the Pt distribution across the film. One exception are the films where tetrachloroplatinate was incorporated. In this case the catalytic activity was tested on films of only 3.5 mm thickness on account of the very low current densities at the electropolymerisation potential. Because of the continuous polymerisation of pyrrole in the solution phase, prolonged film growth was not possible. A glassy carbon (GC) rotating disk electrode (RDE) of 0.28 cm − 2 surface area served as the substrate. Scanning electron microscopy SEM was performed using a Philips microscope type XL-40 with EDX option from EDAX company for the elemental analysis. Samples, in the following referred to as Pt/PPy(I), were prepared by cathodic Pt deposition onto a presynthesised PPy film. The films were obtained from 0.1 M Py+0.1 M NaCl solution. Platinum was deposited from 1 mM H2PtCl6 +0.5 M H2SO4 solution. The following three deposition modes were applied: potentiostatic mode (samples denoted as Pt/PPy(Ia)) double potential step mode (samples denoted as Pt/PPy(Ib)) galvanostatic mode (samples denoted as Pt/PPy(Ic)). Both, the PPy film formation and Pt deposition were carried out in deaerated solution purged with nitrogen. The Pt load was calculated from the charge integral of the cathodic deposition process. Electrodeposition from [PtCl6]2 − solution was assumed to proceed according to
[PtCl6]2 − + 4e− Pt0 + 6Cl− E 0 = 0.744 V vs. SHE
(1)
with an efficiency depending on the cathodic deposition potential. In case of the double potential step technique (mode Ib), the Pt load was estimated on the basis of the cathodic charge difference found between potential pulsing of the PPy film in the absence and presence of [PtCl6]2 − in 0.5 M H2SO4 solution. For a PPy film of approx. 9 mm thickness, the charge corresponding to the cathodic reduction of the PPy film exceeded that for the Pt deposition by about two orders of magnitude. Therefore, the Pt loads calculated in this way may be considered as approximate values only. Samples, in the following referred to as Pt/PPy(II), were prepared by polymerisation of the PPy film from colloidal solutions of nano-scale Pt particles prepared according to Bose and Rajeshwar [19]. The required amount of H2PtCl6 was added to 6.5 cm3 sodium hydrogen citrate solution. The concentration of the sodium hydrogen citrate is strongly dependent on the amount of added hexachloroplatinate acid. This is in agreement with the observation of other authors [19]. Besides the positive action of the citrate, reducing [PtCl6]2 − to Pt0, inhibition effects are prominent. If citrate is present in excess, the Py polymerisation is supposed to decline due to citrate adsorption and blocking of active sites on the anode substrate. The kinetics of charge transfer between [PtCl6] 2 − and Py is also expected to be affected leading to a slowing down of the homogeneous polymerisation of Py. To avoid PPy formation in the bulk solution, citrate at a concentration of 11.5 mM had to be used for the 3 mM H2PtCl6 solution. In the case of the 0.4 mM H2PtCl6 solution a concentration of 1.9 mM citrate was sufficient (concentrations are given with respect to the final volume of the mixture, i.e. 10 ml). Subsequently this solution was placed in an ultrasonic bath for 7 min before 1 cm3 of 1 M NaCl and 2.5 cm3 of 0.2 M Py solutions were added and electropolymerisation of the PPy film was initiated immediately. In order to obtain reproducible results, the surface of the GC electrode had been covered prior to electropolymerisation by a thin layer (0.5 mm) of pure PPy [19]. Samples, in the following referred to as Pt/PPy(III), were prepared by electropolymerisation of PPy from a solution of 0.05 M pyrrole and 10 mM K2PtCl4. After electropolymerisation the samples were placed in 0.5 M H2SO4 and cycled 40 times between + 0.6 and − 0.6 V at a scan rate of 50 mV s − 1 to reduce the tetrachloroplatinate in the PPy film according to Eq. (2). Hepel et al. [13,20] proposed to cycle between +0.6 and − 1.3 V. In that case intensive hydrogen evolution and mechanical damage of the composite films were observed in the negative potential region.
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1 1 1 1 = + + jt jK j lfilm j dif l
[PtCl4]2 − +2e− Pt0 +4Cl− E 0 =0.758 V vs. SHE
(2)
The electrocatalytic properties of the composite film electrodes were tested for the anodic hydrogen oxidation reaction in 0.5 M H2SO4 under H2 atmosphere at room temperature. Steady state polarisation curves were measured potentiostatically always using the same polarisation routine. The potential was changed in 0.1 V steps from −0.3 to 0.6 V and the corresponding current was taken after stabilisation. The potential range studied was selected in order to monitor the potential of commencement of the hydrogen oxidation reaction and the degree of inhibition of this reaction at the more anodic potentials. Since the current density used for the analysis is taken at the fixed potential, 100 mV potential steps assured sufficient accuracy. A rotating disk electrode RDE was used to establish controlled convection.
3. Data analysis The total current density jt of the RDE can be separated into a kinetic jk and mass transport controlled contribution jl 1 1 1 = + jt jk jl
(3)
The simplest case is the totally mass-transfer controlled electrode reaction, the limiting current density of which is described by the Levich–equation 1/2 − 1/6 jl =0.62 n F A D2/3 w c*H H
(6)
Since the rate of the reaction increases exponentially with increasing overvoltage, the kinetic contribution 1/jk in Eq. (6) tends to zero at sufficiently high overvoltage. What remains are the transport controlled contributions, which can be separated experimentally as diffusion limited current density j dif depending on the l rotation frequency , cf. Eq. (4), and the term j film , l which is independent of . Therefore, at sufficiently high overpotentials, 1/j film can be obtained as intercept l on the abscissa from extrapolation of the Koutecky– Levich plot at . However, this is valid only for a homogeneously active electrode surface. If only a part of the electrode surface is electrochemically active (e.g. a composite film with a low Pt load), the surface concentration of electroactive species never reaches zero. It is because of the enrichment of the depleted surface solution layer by diffusion of the electroactive species from the bulk of the electrolyte during the period when it is in contact with the electrochemically inactive part of the electrode surface. This results in an apparent kinetic control of the electrode process. Landsberg and Thiele [24] studied the influence of partial inactivation of RDE on the kinetics of mass transfer controlled reactions. The effect of partially inactive electrodes was also analysed by impedance measurements and discussed in terms of a modified transfer function [25,26].
4. Results and discussion
(4)
where n, F, A and D have their usual electrochemical meaning, is the angular rotation frequency, w is the kinematic viscosity of the fluid and c* is the bulk concentration of the reactant, in our case hydrogen. By introducing Eq. (4) into Eq. (3) the following relation is obtained 1 1 w 1/6 = + 1/2 jt jk 0.62 n F A D2/3 c*H H
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(5)
As follows from Eq. (4), the totally mass-transfer limited current density is linearly dependent on 1/2 with zero current without rotation. Kinetic limitation of the electrode reaction results in a curvature of the Levich plot ( j vs. 1/2) or, more clearly, according to Eq. (5) in a finite intercept on the abscissa at in the inverse Levich plot ( j − 1 vs. − 1/2), also denoted Koutecky–Levich plot. At conducting polymer film covered electrodes, where different rate and mass transport limited steps have been identified, the situation is more complex, as discussed already by Croissant et al., [23]. In particular, limited diffusion current densities of molecular hydrogen in the bulk electrolyte ( j dif l ) and in the film ( j film ) can be distinguished. l
4.1. Platinum distribution and catalytic acti6ity of Pt/PPy(I) A series of experiments was carried out to deposit platinum onto pre-synthesised PPy films using potentiostatic (Ia), double potential step (Ib), and galvanostatic (Ic) modes of deposition. At first the efficiency of Pt deposition by cathodic reduction of [PtCl6]2 − as function of the applied potential was studied. The amount of deposited Pt was roughly determined by EDX analysis. The intensity of the Pt signal at constant cathodic charge was found to increase when the potential was decreased from 0.3 to − 0.1 V. At more negative potentials (EB − 0.1 V) the EDX signal intensity did not further increase. There are two possible reasons for such behaviour. Depending on the potential applied, platinum is deposited at different depths of the film and is partially not accessible to EDX surface analysis. Alternatively, the deposition process at less cathodic potentials (E\ − 0.1 V) does not proceed with 100% current efficiency due to side reactions, e.g. [PtCl6]2 − reduction to [PtCl4]2 − according to the reaction
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Fig. 1. Cross-section SEM of the Pt/PPy(Ia) film with 0.6 mg cm − 2 Pt load; deposition mode: potentiostatic at E= + 0.2 V.
Fig. 3. Cross-section SEM of the Pt/PPy(Ic) film with 1.0 mg cm − 2 calculated Pt load; deposition mode: galvanostatic with j = −0.1 mA cm − 2.
[PtCl6]2 − +2e− [PtCl4]2 − +2Cl− E 0 =0.726 V vs. SHE
(7)
As shown below, the later explanation seems to be more probable. The efficiency of the Pt deposition at − 0.1 V was considered to be 100%. However, the final preparation of Pt/PPy(Ia) samples using potentiostatic deposition mode (Ia) was carried out at a more positive potential E= +0.2 V. This value ensures both, the necessary cathodic polarisation for the reduction of platinum and sufficient electronic conductivity of PPy. The relatively slow process of Pt deposition at this potential also provides a better condition for the diffusion of [PtCl6]2 − into the PPy film. Under these conditions a current efficiency of 50% was estimated for the Pt deposition process. Samples with
Fig. 2. Cross-section SEM of the Pt/PPy(Ib) film with 0.6 mg cm − 2 calculated Pt load; deposition mode: double potential step with anodic potential E = +0.5 V (10 s); cathodic potential E = −0.1 V (1 s).
different platinum load ranging between 0.13 and 2.9 mg cm − 2 were obtained by varying the deposition time. The SEM micrograph in Fig. 1 shows the cross-section of a Pt/PPy(Ia) sample with a load of 0.6 mg Pt cm − 2. The deposited Pt can be seen on top of the surface. The double potential step method (mode Ib) was used to improve the penetration of the [PtCl6]2 − complex into the PPy film. This was believed to result in a three-dimensional Pt catalyst distribution. The procedure consisted of a periodical sequence of anodic and cathodic potential steps applied to the PPy film in hexachloroplatinate solution. The anodic potential was fixed at 0.5 V. The cathodic potential varied between + 0.2 and −0.5 V. The ratio of the anodic and cathodic pulse period, ~A/~C, was varied from 10 s / 1 s to 5 s / 5 s and the calculated Pt load varied from 0.05 to 2.1 mg cm − 2, respectively. Fig. 2 shows a typical SEM picture of the cross-section of a Pt/PPy(Ib) sample prepared by this method with a calculated Pt load of 0.6 mg cm − 2. Finally, galvanostatic modification of the PPy layer was performed to obtain samples Pt/PPy(Ic). A GC electrode covered with a PPy film was polarised with a cathodic current density of −0.1 mA cm − 2. On varying the polarisation time, different Pt load was obtained ranging between 0.2 and 1.0 mg Pt cm − 2. The SEM picture in Fig. 3 shows a cross-section of such a film with a Pt load of 1.0 mg cm − 2. Because of the relatively small deposition current density the actual electrode potential was not below + 0.2 V, so that under these conditions the parasitic reaction described by Eq. (7) prevailed. This was also indicated by the colour of the hexachloroplatinate solution, which changed from yellowish to orange during the deposition process. Under these conditions a current efficiency of 20% was estimated for the Pt deposition process.
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Fig. 4. Ratio of the Pt to Carbon EDX signal intensities as function of the distance from the film surface; film type and Pt load: O-Pt/PPy(Ia), 0.6 mg cm − 2; 9-Pt/PPy(Ib), 0.6 mg cm − 2,
-Pt/PPy(Ic), 1.0 mg cm − 2; -Pt/PPy(II), 1.8 mM [PtCl6] 2 − in the synthesis solution; D-Pt/PPy(III), 10 mM K2PtCl4 in the synthesis solution.
EDX spectra were taken from the surface and from different locations within the composite film (cross-section). Fig. 4 shows the ratio of the Pt to carbon signal intensity as a function of the distance from the film surface for the different samples prepared. It can be
Fig. 5. Cyclic voltammograms of a Pt/PPy(Ib) sample prepared as descibed in Fig. 2; Pt load 2.1 mg cm − 2, electrode rotation rate 1000 RPM, potential scan rate 20 mV s − 1, electrolyte 0.5 M H2SO4 bubbled with N2. The arrow indicates the change of the peak current with increasing number of cycles.
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clearly seen that the intensity of the Pt:C signal decreases rapidly in the film interior for all modes of Pt deposition on the pre-synthesised PPy film (Ia– Ic). The EDX signal of Pt in the film interior can result either from platinum in the reduced metallic state Pt0 or from the [PtCl6]2 − complex, present as counter ion. There is no direct evidence in our work, but it is more likely that most of the Pt is present in the oxidised form. This was supported by the following experiment. EDX analysis of Pt/PPy(Ib) samples were prepared before and after cyclic voltammetry in 0.5 M H2SO4 under N2 atmosphere. The Pt:C peak ratio of a freshly prepared film measured 0.5 mm inside the film was found to be 0.60 indicating a significant amount of Pt. After cyclic voltammetry the peak ratio decreased to 0.23. The corresponding voltammograms in Fig. 5 exhibit the appearance of a strong cathodic peak with decreasing peak current from cycle to cycle. This may be explained by the reduction of Pt initially present in the form of [PtCl6]2 − , moving from the interior of the film to the film/solution interface, where it is either reduced to Pt0 or dissolved in the bulk of the electrolyte solution. The actual potential of [PtCl6]2 − reduction in solution is approximately 0.5 V [27], whereas the cathodic peak in Fig. 5 appears at more negative potentials around − 0.25 V. This can be explained by a kinetic hindrance of the [PtCl6]2 − reduction in the interior of the PPy film. According to Eq. (1), reduction of the complex would release six chloride ions, which would have to be removed very fast from the film interior to keep local electroneutrality. On the other hand, by reducing the PPy film, negative counter ions (amongst others [PtCl6]2 − ) are transported to the film/ solution interface, where the [PtCl6]2 − complex can be reduced immediately. That is why the Pt reduction peak in Fig. 5 appears directly after the maximum of the PPy reduction peak at 0 V. This all indicates that Pt is predominantly deposited on top of the PPy film. The catalytic properties of the composite films were tested by steady state polarisation curves of the hydrogen oxidation. A set of polarisation curves at different rotation rates is shown in Fig. 6 for electrodes prepared by deposition modes (Ia– Ic) and for a bare platinum electrode Pt. Compared with bare platinum, the polarisation curves of the composite electrodes exhibit higher overvoltage. The rising part of these curves is obviously controlled by the degree of oxidation and the corresponding electronic conductivity of the PPy matrix at low potentials. The values of the transport controlled limiting current densities are higher compared with the bare platinum electrode. The well known decrease of the limiting current density on bare platinum at anodic potentials due to the formation of a PtO layer is less pronounced on samples (Ia) and (Ic) and almost absent on sample (Ib). The reason is probably identical to that of the increased resistance of the Pt/PPy composite
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Fig. 7. Koutecky– Levich plots of the hydrogen oxidation current density at E= +0.2 V on Pt/PPy(Ia) electrodes of different Pt load: O-0.1 mg cm − 2, 9-0.2 mg cm − 2, -0.6 mg cm − 2 and -3.0 mg cm − 2; dashed line represents bare Pt electrode at E= − 0.1 V; electrolyte 0.5 M H2SO4 +H2(1 atm).
potentials, the reaction proceeds under the control of pure mass transfer from the solution to the film surface and presumably by diffusion within the film. Kinetic control can be neglected, except for the samples with low Pt load, where a few Pt islands only cover the surface, as discussed above (partially active electrode).
Fig. 6. Potentiostatic polarisation curves of hydrogen oxidation on Pt/PPy(Ia– Ic) and bare Pt electrodes (RDE) in 0.5 MH2SO4 + H2(1 atm); rotation rate/RPM: O-50, 9-100, 200, -500, D-1000, -2000 and -3000. Pt load: Pt/PPy(Ia) 0.6 mg cm − 2, Pt/PPy(Ib) 0.6 mg cm − 2 and Pt/PPy(Ic) 0.7 mg cm − 2.
against the poisoning of CO [10,11,13,17], i.e. up to now unspecified interactions between conducting polymer and Pt microparticles. Similar sets of polarisation curves were measured on samples Pt/PPy(Ia–Ic) prepared with different platinum loads. From the transport controlled part of these polarisation curves the Koutecky–Levich plots were obtained at a fixed electrode potential E =0.2 V (Figs. 7–9), except for bare Pt with E= −0.1 V. At these
Fig. 8. Koutecky– Levich plots of the hydrogen oxidation current density at E= +0.2 V on Pt/PPy(Ib) electrodes of different Pt load: O-0.2 mg cm − 2, 9-0.4 mg cm − 2, -0.6 mg cm − 2 and -2.1 mg cm − 2; dashed line represents bare Pt electrode at E= − 0.1 V; electrolyte 0.5 M H2SO4 +H2(1 atm).
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Fig. 9. Koutecky– Levich plots of the hydrogen oxidation current density at E = +0.2 V on Pt/PPy(Ic) electrodes of different Pt load: O-0.2 mg cm − 2, 9-0.7 mg cm − 2 and -1.0 mg cm − 2; dashed line represents bare Pt electrode at E= − 0.1 V; electrolyte 0.5 M H2SO4 + H2(1 atm).
Corresponding to this fact are the intercepts of the linear regression lines with the y–axis at . The reciprocal diffusion limited current density, 1/j film , obl tained from the intercept according to Eq. (6), was plotted on a logarithmic scale versus the platinum load, Fig. 10. For the samples (Ia) and (Ib) the decrease of 1/j film with increasing platinum load exhibits two rel gions of different slope. For sample (Ic) this behaviour is not as clear, probably due to the narrow range of Pt loads studied and the limited number of experiments.
Fig. 10. Limiting diffusion current density (1/j film ) obtained l from the intercept of the Koutecky–Levich plots (Figs. 7–9) as function of the platinum load for different sample electrodes: O-Pt/PPy(Ia), 9-Pt/PPy(Ib) and -Pt/PPy(Ic).
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Such behaviour indicates that the process is controlled by two different mechanisms. At Pt loads below approx. 0.5 mg cm − 2 (depending on the Pt deposition mode), the main contribution to the finite intercept originates from the apparent kinetic control of the electrode reaction caused by the low film surface coverage by Pt microparticles. At Pt loads above 0.5 mg cm − 2, the intercept may be referred to the diffusion of hydrogen into the film interior (1/j film ). This increase of l the diffusion rate with the platinum load indicates that the characteristic diffusion length becomes shorter if the density of active platinum sites in the film increases. The data obtained are comparable to those reported by Croissant et. al., [23]. He explains the limitation of the j film at the Pt loads below 0.25 mg cm − 2 by the hydrogen adsorption limiting current. From Figs. 7 – 9 it is also evident that the limiting current densities of the composite electrodes prepared by cathodic deposition of Pt on PPy films are higher compared to the bare Pt electrode and their slope depends on the Pt load. This can be explained by the surface roughness of the PPy film causing more intensive mass transfer between the bulk of the electrolyte and the electrode surface compared to the polished bare Pt electrode. The difference in the slope of the Koutecky– Levich plots originates from the degree of surface coverage of Pt and/or diffusion of hydrogen into the film interior.
4.2. Platinum distribution and catalytic acti6ity of Pt/PPy(II) Samples were prepared by electropolymerisation of the PPy film from colloidal solutions of nano-scale Pt particles. The concentration of the hexachloroplatinate complex added was varied and the amount of Pt incorporated and its effect on the catalytic activity was studied. As demonstrated in Fig. 11 metallic Pt0 particles cannot be observed by SEM. However, the EDX spectra showed a strong and uniform Pt signal over the whole cross section of the PPy film. The ratio of the EDX signal intensity of Pt and carbon is shown in Fig. 4. Furthermore, the relative signal intensity of platinum was found to be proportional to the hexachloroplatinate concentration in the synthesis solution. According to Bose and Rajeshwar [19], the PPy films prepared in this way should contain platinum exclusively in the form of colloidal metallic particles. This statement was supported by XPS surface analysis indicating the presence of Pt0 only. Although not explicitly denoted by the authors, the spectrum was most probably taken from the surface of the sample. By XPS analyses very thin surface layer is probed (approx. 10 nm). However, this layer is strongly influenced by the polarisation procedure during the synthesis or after-
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wards (potential cycling between +0.95 and −0.80 V [19]). Platinum is easily reduced to the metallic form especially at the film surface. In our opinion, inside the PPy film platinum is present in both forms. As already mentioned before, citrates not only reduce [PtCl6]2 − to the form of colloidal Pt0 particles, but also inhibit the charge transfer between the [PtCl6]2 − complex and Py. This means, that to a certain extent [PtCl6]2 − is present in the synthesis solution during electropolymerisation of the PPy film. As shown by Hyodo and Omae [28], large anions are selectively incorporated as counter ions during synthesis of the film. An indication for the presence of [PtCl6]2 − in the film interior was the appearance of a cathodic current peak in the cyclic voltammograms obtained immediately after the electropolymerisation, similar as discussed above in the Pt/ PPy(Ib) system. Moreover, the increase in the Pt signal intensity of the EDX spectra (c.f. Fig. 4) with increasing distance from the film surface indicates that part of the Pt is removed from the interior of the film and is probably replaced by sulphate ions during cyclovoltammetric testing. Potentiostatic polarisation curves of Pt/PPy(II) composite films and the corresponding Koutecky–Levich plots for different Pt loads are shown in Fig. 12. For all Pt concentrations studied, extrapolation leads to near zero intercepts on the 1/j axis; exact values are difficult to obtain because of the slight curvature of the plots. The following data were obtained using linear regression: 0.44 mA − 1 cm2 (with 0.4 mM [PtCl6]2 − in the synthesis solution), 0.28 mA − 1 cm2 (with 1.8 mM [PtCl6]2 − ) and 0.28 mA − 1 cm2 (with 3.0 mM [PtCl6]2 − ). The low values of the intercepts indicate that the reaction proceeds predominantly on the film surface relatively rich on Pt particles.
Fig. 11. Cross-section SEM of the Pt/PPy(II) film synthesised from Pt dispersion solution containing 1.8 mM [PtCl6] 2 − .
Fig. 12. (a) Potentiostatic polarisation curves of hydrogen oxidation on Pt/PPy(II) synthesised from Pt dispersion solution containing 1.8 mM [PtCl6] 2 − ; rotation rate/RPM: O-50, 9-100, -200, -500, D-1000, -2000 and -3000; electrolyte 0.5 M H2SO4 +H2(1 atm). (b) Koutecky– Levich plots of hydrogen oxidation on Pt/PPy(II) synthesised from Pt dispersion solution of different [PtCl6] 2 − concentration: O-0.4 mM, 9-1.8 mM and -3.0 mM; dashed line bare Pt.
The fact that the Koutecky– Levich plots of the two samples prepared from solutions containing 1.8 and 3.0 mM [PtCl6]2 − are almost identical may serve as indirect evidence that the hydrogen oxidation is limited by the amount of Pt on the surface and not in the interior of the film. In the case of the 3.0 mM [PtCl6]2 − solution a Pt EDX signal of similar intensity was recorded from the surface of the film and even a stronger one from inside the film. It can be concluded that, if H2 diffusion occurs into the film interior, it is restricted to a depth of approx. 1 mm (the thickness of the samples studied by EDX).
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4.3. Platinum distribution and catalytic acti6ity of Pt/PPy(III) In the case of potentiostatic polymer film formation at a given potential (E =0.75 V) the anodic current reached a value about one order of magnitude lower than for the previously discussed methods. Since, after prolonged electropolymerisation, PPy formation in the bulk of the solution (darkening) becomes apparent, PPy layers with a thickness of only 3.5 mm were synthesised by this method. Fig. 13 shows an SEM picture of the cross-section of the film synthesised from solution containing 10 mM K2PtCl4. It is not possible to identify individual Pt0 particles distributed in the film by SEM. However, the strong EDX signal of Pt shown in Fig. 4 proves again that Pt is present in the film. It is clear from EDX analysis, that the concentration of Pt in the film interior is several times higher compared to a film synthesised from solution containing colloidal Pt particles. The presence of Pt in the form of crystallites of approximately 10 nm diameter in a PPy film prepared in a similar way was proven by Hepel [13] using transmission electron microscopy. This is apparently in contradiction to the interpretation of the changes observed in the EDX spectra of the Pt/PPy(Ib) samples after cyclic polarisation. However, the situation in both systems is quite different. In the case of the Pt/PPy(III) electrode, the tetrachloroplatinate complex is incorporated during electropolymerisation and according to EQCM experiments of Hepel et al., [13,20] it is fixed in the polymer structure and cannot move out of the film. Moreover, six chloride ions are released during the reduction of the hexachloroplatinate in the case of Pt/PPy(Ib) formation whereas only four are released in the case of Pt/PPy(III). This also causes an important
Fig. 13. Cross-section SEM of the Pt/PPy(III) film electropolymerised from solution containing 10 mM K2PtCl4.
Fig. 14. (a) Potentiostatic polarisation curves of hydrogen oxidation on Pt/PPy(III) synthesised from solution containing 10 mM [PtCl4]2 − , rotation rate/RPM: O-50, 9-100, -200, -500, D-1000, -2000 and -3000; electrolyte 0.5 M H2SO4 +H2(1 atm). (b) Koutecky– Levich plot of the polarisation curves in 14a at E= +0.2 V; dashed line represents the bare Pt electrode.
difference in the preference of the cathodic reduction of the complexes. The polarisation curves of hydrogen oxidation of the Pt/PPy(III) system and the corresponding Koutecky– Levich plots shown in Fig. 14 exhibit a behaviour which differs considerably from that of the previously discussed samples. The polarisation curves (14a) exhibit extremely low overvoltages and the limiting current densities are reached at about E= − 0.2 V remaining constant over the whole potential range explored. The transition of the PPy matrix to the conducting state is not observed. This is in agreement with the crystal microbalance measurements of Hepel [13], according to which ion exchange does not occur and pseudocapaci-
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tance peaks are absent in the characteristics of the potentiodynamic polarisation curve on this type of PPy composite electrodes. According to Hepel, the material does not any more behave like PPy but more like bare Pt. In our case the electrocatalytic activity of the Pt/ PPy(III) composite electrodes was several times lower than that of bare platinum. This is quite surprising for a film of such high Pt content homogeneously distributed in its structure. The high value of the 1/j intercept, 5.80 mA − 1 cm2, of the Koutecky–Levich plots indicate that hydrogen oxidation kinetics must be predominantly controlled by a process different from the mass transfer to the film surface. Further investigations are necessary to understand this phenomenon.
5. Conclusions In the present study it was shown that a real three-dimensional homogeneous distribution of Pt catalyst microparticles inside the conducting polymer film cannot be obtained by cathodic deposition of platinum on a pre-synthesised polypyrrole film. To achieve this, platinum has to be incorporated into the polymer film during electropolymerisation either in the form of colloidal particles or in the form of counter ions, which can be reduced subsequent to electropolymerisation by an appropriate cathodic treatment. The highest Pt loads inside the film were obtained by incorporation of [PtCl4]2 − as counter ion during synthesis of the film and its subsequent reduction. However, such films manifest very low catalytic activity for the hydrogen oxidation reaction in comparison to the films prepared by the alternative methods. Polypyrrole films modified by cathodic platinum deposition on presynthesised polypyrrole film, or platinum incorporation during polymerisation in the form of colloidal particles exhibit electrocatalytic activities comparable to that of bare Pt electrodes. The deposition potential of Pt was found to be an important parameter with respect to the efficiency of the deposition process. There was little evidence of anodic hydrogen oxidation proceeding inside the three-dimensional structure of the composite Pt/PPy catalyst layers.
Acknowledgements The authors are indebted to the Deutsche Forschungsgemeinschaft (DFG) for financial support of this work within the frame of the Schwerpunktprogramm ‘Neuartige Schichtstrukturen fu¨r Brennstoffzellen’ under contract No. JU 201/6. One of us (K.B.) is grateful to the Alexander von Humboldt Foundation for making this co-operation possible.
References [1] T.A. Skotheim (Ed.), Handbook of Conducting Polymers, vols. 1 – 2, Marcel Decker, New York, 1986. [2] C.C. Chen, C.S.C. Bose, K. Rajeshwar, J. Electroanal. Chem. 350 (1993) 161. [3] C.T. Hable, M.S. Wrighton, Langmuir 7 (1991) 1305. [4] M.A. Del Valle, F.R. Dı´az, M.E. Bodini, T. Pizarro, R. Co´rdova, H. Go´mez, R. Schrebler, J. Appl. Electrochem. 28 (1998) 943. [5] A. Kelaidopoulou, E. Abelidou, A. Papoutsis, E.K. Polychroniadis, G. Kokkinidis, J. Appl. Electrochem. 28 (1998) 1101. [6] F. Ficicioglu, F. Kardirgan, J. Electroanal. Chem. 451 (1998) 95. [7] P.J. Kulesza, M. Matczak, A. Wolkiewicz, B. Grzybowska, M. Galkowski, M.A. Malik, A. Wieckowski, Electrochim. Acta 44 (1999) 2131. [8] F.T.A. Vork, L.J.J. Janssen, E. Barendrecht, Electrochim. Acta 31 (1986) 1569. [9] M. Gholamian, A.Q. Contractor, J. Electroanal. Chem. 289 (1990) 69. [10] D.J. Strike, N.F. De Rooij, M. Koudelka-Hep, M. Ulmann, J. Augustynaski, J. Appl. Electrochem. 22 (1992) 922. [11] H. Laborde, J.-M. Leger, C. Lamy, J. Appl. Electrochem. 24 (1994) 219. [12] H. Laborde, J.-M. Leger, C. Lamy, J. Appl. Electrochem. 24 (1994) 1019. [13] M. Hepel, J. Electrochem. Soc. 145 (1998) 124. [14] F.T.A. Vork, L.J.J. Janssen, E. Barendrecht, Electrochim. Acta 32 (1987) 1187. [15] F.T.A. Vork, E. Barendrecht, Electrochim. Acta 35 (1990) 135. [16] S. Holdcroft, L. Funt, J. Electroanal. Chem. 240 (1988) 89. [17] K.M. Kost, D.E. Bartak, B. Kazee, T. Kuwana, Anal. Chem. 60 (1988) 2379. [18] P.O. Esteban, J.-M. Leger, C. Lamy, E. Genies, J. Appl. Electrochem. 19 (1989) 462. [19] C.S.C. Bose, K. Rajeshwar, J. Electroanal. Chem. 333 (1992) 235. [20] M. Hepel, M. Yi-Chen, U. Stimmings, New Materials for Fuel Cell Systems I, in: O. Savadogo, P.R. Roberge, T.N. Veziroglu (Eds.), Les E´ditions de E´cole Polytechnique de Montre´al, Collection, Actes de colloque Montre´al, Que´bec, Canada, 1995 pp. 629– 657. [21] E.K. Lai, P.D. Beattie, E Simon, S. Holdcroft, Synth. Met. 84 (1997) 87. [22] S. Basak, K. Rajeshwar, M. Kaneko, Anal. Chem. 62 (1990) 1407. [23] M.J. Croissant, T. Napporn, J.-M. Le´ger, C. Lamy, Electrochim. Acta 43 (1998) 2447. [24] R. Landsberg, R. Thiele, Electrochim. Acta 11 (1966) 1243. [25] J. Hitzig, J. Titz, K. Ju¨ttner, W.J. Lorenz, Electrochim. Acta 29 (1984) 287. [26] E. Schmidt, J. Hitzig, K. Ju¨ttner, W.J. Lorenz, Electrochim. Acta 31 (1986) 1041. [27] A.J. Bard, R. Parsons, J. Jordan, Standard Potentials in Aqueous Solution, Marcel Dekker, New York, 1985. [28] K. Hyodo, M. Omae, Electrochim. Acta 35 (1990) 1245.
Platinum distribution and electrocatalytic properties of modified polypyrrole films K. Bouzek 1,2, K.-M. Mangold, K. Ju¨ttner *,1 Karl-Winnacker-Institut der DECHEMA e.V., Theodor-Heuss-Allee 25, D-60486 Frankfurt am Main, Germany Received 12 April 2000; received in revised form 18 August 2000
Abstract For the preparation and modification of conducting polymer films as electrocatalysts for anodic hydrogen oxidation three pathways were investigated: (i) cathodic deposition of platinum from a hexachloro-platinum complex [PtCl6]2 − on the pre-synthesised polymer film, (ii) incorporation of colloidal platinum particles into the polymer film during electropolymerisation and (iii) incorporation of a tetrachloro-platinum complex [PtCl4]2 − during the electropolymerisation as counter ion and its subsequent cathodic reduction. Using SEM and EDX the distribution of platinum in the polypyrrole film depending on the preparation conditions was studied. The catalytic activity of the modified films was derived from polarisation curves of hydrogen oxidation using rotating disk electrode technique. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: Polypyrrole; Platinum; Distribution; Electrocatalysis; Hydrogen oxidation
1. Introduction Conducting polymers have been studied intensively in recent years because of their possible utilisation in many practical applications, such as charge storage devices, electrochromic displays, modified electrodes, anti-corrosion coatings, molecular transistors and high surface area support for catalyst microparticles [1]. The incorporation of catalyst microparticles into a conducting polymer film represents an attractive field of research. The catalytic activity of platinum incorporated into a conducting polymer layer for the oxidation of hydrogen or for small organic molecules has been the subject of previous investigations [2–13]. Polypyrrole and polyaniline are the most common con* Corresponding author. Fax: +49-6975-64388. 1 ISE member. 2 On leave from the Department of Inorganic Technology, Institute of Chemical Technology, Technicka´, 166 28 Prague 6, Czech Republic.
ducting polymers in these studies. Several ways of preparing platinum modified conducting polymer films have been reported in the literature. They can be divided into three main groups: (i) electrochemical Pt deposition onto the previously synthesised polymer film [4 – 18], (ii) incorporation of colloidal Pt particles prepared by chemical reduction of hexachloroplatinate during electropolymerisation of the film [2,19] and (iii) incorporation of tetrachloroplatinate as a counter ion and its subsequent cathodic reduction after electropolymerisation of the polymer film [13– 17,19,20]. Most authors suppose the platinum microparticles to be homogeneously dispersed over the conducting polymer film [4,7,8,10,11,13– 17,19,20]. In a few papers a two-dimensional distribution is indicated if the platinum particles were cathodically deposited on the presynthesised polymer film [13,16,21]. It is the aim of this work to compare the above-mentioned methods with respect to the distribution of platinum in the polymer film and the catalytic activity of the composite layer for the hydrogen oxidation reaction.
0013-4686/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 0 0 ) 0 0 6 5 9 - 9
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2. Experimental Electrochemical measurements were performed with a HEKA Potentiostat-Galvanostat (PG310) using a three-electrode configuration in a double compartment glass cell. All potentials refer to the SCE if not otherwise indicated. The nozzle of the Luggin capillary is filled with a magnesium rod in order to minimise the diffusion of Cl− ions into the electrolyte solution and to avoid poisoning of the Pt modified electrode. The chemicals were commercial materials of the highest available purity and were used without further purification with the exception of pyrrole Py, which was purified by distillation. In all cases the polypyrrole films (PPy) were synthesised potentiostatically at 0.75 V, this is close to the potential used by Bose and Rajeshwar [19] and reported to correspond to approximately 90% efficiency of PPy electropolymerisation. The temperature was kept constant at 20°C, the thickness of the films was controlled by the charge flowing during the anodic electropolymerisation. A charge of 100 mC cm − 2 was assumed to correspond to a film thickness of 0.25 mm [10,22]. Compared with previous investigations, relatively thick films of typically 9 91 mm were synthesised to permit a more detailed study of the Pt distribution across the film. One exception are the films where tetrachloroplatinate was incorporated. In this case the catalytic activity was tested on films of only 3.5 mm thickness on account of the very low current densities at the electropolymerisation potential. Because of the continuous polymerisation of pyrrole in the solution phase, prolonged film growth was not possible. A glassy carbon (GC) rotating disk electrode (RDE) of 0.28 cm − 2 surface area served as the substrate. Scanning electron microscopy SEM was performed using a Philips microscope type XL-40 with EDX option from EDAX company for the elemental analysis. Samples, in the following referred to as Pt/PPy(I), were prepared by cathodic Pt deposition onto a presynthesised PPy film. The films were obtained from 0.1 M Py+0.1 M NaCl solution. Platinum was deposited from 1 mM H2PtCl6 +0.5 M H2SO4 solution. The following three deposition modes were applied: potentiostatic mode (samples denoted as Pt/PPy(Ia)) double potential step mode (samples denoted as Pt/PPy(Ib)) galvanostatic mode (samples denoted as Pt/PPy(Ic)). Both, the PPy film formation and Pt deposition were carried out in deaerated solution purged with nitrogen. The Pt load was calculated from the charge integral of the cathodic deposition process. Electrodeposition from [PtCl6]2 − solution was assumed to proceed according to
[PtCl6]2 − + 4e− Pt0 + 6Cl− E 0 = 0.744 V vs. SHE
(1)
with an efficiency depending on the cathodic deposition potential. In case of the double potential step technique (mode Ib), the Pt load was estimated on the basis of the cathodic charge difference found between potential pulsing of the PPy film in the absence and presence of [PtCl6]2 − in 0.5 M H2SO4 solution. For a PPy film of approx. 9 mm thickness, the charge corresponding to the cathodic reduction of the PPy film exceeded that for the Pt deposition by about two orders of magnitude. Therefore, the Pt loads calculated in this way may be considered as approximate values only. Samples, in the following referred to as Pt/PPy(II), were prepared by polymerisation of the PPy film from colloidal solutions of nano-scale Pt particles prepared according to Bose and Rajeshwar [19]. The required amount of H2PtCl6 was added to 6.5 cm3 sodium hydrogen citrate solution. The concentration of the sodium hydrogen citrate is strongly dependent on the amount of added hexachloroplatinate acid. This is in agreement with the observation of other authors [19]. Besides the positive action of the citrate, reducing [PtCl6]2 − to Pt0, inhibition effects are prominent. If citrate is present in excess, the Py polymerisation is supposed to decline due to citrate adsorption and blocking of active sites on the anode substrate. The kinetics of charge transfer between [PtCl6] 2 − and Py is also expected to be affected leading to a slowing down of the homogeneous polymerisation of Py. To avoid PPy formation in the bulk solution, citrate at a concentration of 11.5 mM had to be used for the 3 mM H2PtCl6 solution. In the case of the 0.4 mM H2PtCl6 solution a concentration of 1.9 mM citrate was sufficient (concentrations are given with respect to the final volume of the mixture, i.e. 10 ml). Subsequently this solution was placed in an ultrasonic bath for 7 min before 1 cm3 of 1 M NaCl and 2.5 cm3 of 0.2 M Py solutions were added and electropolymerisation of the PPy film was initiated immediately. In order to obtain reproducible results, the surface of the GC electrode had been covered prior to electropolymerisation by a thin layer (0.5 mm) of pure PPy [19]. Samples, in the following referred to as Pt/PPy(III), were prepared by electropolymerisation of PPy from a solution of 0.05 M pyrrole and 10 mM K2PtCl4. After electropolymerisation the samples were placed in 0.5 M H2SO4 and cycled 40 times between + 0.6 and − 0.6 V at a scan rate of 50 mV s − 1 to reduce the tetrachloroplatinate in the PPy film according to Eq. (2). Hepel et al. [13,20] proposed to cycle between +0.6 and − 1.3 V. In that case intensive hydrogen evolution and mechanical damage of the composite films were observed in the negative potential region.
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1 1 1 1 = + + jt jK j lfilm j dif l
[PtCl4]2 − +2e− Pt0 +4Cl− E 0 =0.758 V vs. SHE
(2)
The electrocatalytic properties of the composite film electrodes were tested for the anodic hydrogen oxidation reaction in 0.5 M H2SO4 under H2 atmosphere at room temperature. Steady state polarisation curves were measured potentiostatically always using the same polarisation routine. The potential was changed in 0.1 V steps from −0.3 to 0.6 V and the corresponding current was taken after stabilisation. The potential range studied was selected in order to monitor the potential of commencement of the hydrogen oxidation reaction and the degree of inhibition of this reaction at the more anodic potentials. Since the current density used for the analysis is taken at the fixed potential, 100 mV potential steps assured sufficient accuracy. A rotating disk electrode RDE was used to establish controlled convection.
3. Data analysis The total current density jt of the RDE can be separated into a kinetic jk and mass transport controlled contribution jl 1 1 1 = + jt jk jl
(3)
The simplest case is the totally mass-transfer controlled electrode reaction, the limiting current density of which is described by the Levich–equation 1/2 − 1/6 jl =0.62 n F A D2/3 w c*H H
(6)
Since the rate of the reaction increases exponentially with increasing overvoltage, the kinetic contribution 1/jk in Eq. (6) tends to zero at sufficiently high overvoltage. What remains are the transport controlled contributions, which can be separated experimentally as diffusion limited current density j dif depending on the l rotation frequency , cf. Eq. (4), and the term j film , l which is independent of . Therefore, at sufficiently high overpotentials, 1/j film can be obtained as intercept l on the abscissa from extrapolation of the Koutecky– Levich plot at . However, this is valid only for a homogeneously active electrode surface. If only a part of the electrode surface is electrochemically active (e.g. a composite film with a low Pt load), the surface concentration of electroactive species never reaches zero. It is because of the enrichment of the depleted surface solution layer by diffusion of the electroactive species from the bulk of the electrolyte during the period when it is in contact with the electrochemically inactive part of the electrode surface. This results in an apparent kinetic control of the electrode process. Landsberg and Thiele [24] studied the influence of partial inactivation of RDE on the kinetics of mass transfer controlled reactions. The effect of partially inactive electrodes was also analysed by impedance measurements and discussed in terms of a modified transfer function [25,26].
4. Results and discussion
(4)
where n, F, A and D have their usual electrochemical meaning, is the angular rotation frequency, w is the kinematic viscosity of the fluid and c* is the bulk concentration of the reactant, in our case hydrogen. By introducing Eq. (4) into Eq. (3) the following relation is obtained 1 1 w 1/6 = + 1/2 jt jk 0.62 n F A D2/3 c*H H
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(5)
As follows from Eq. (4), the totally mass-transfer limited current density is linearly dependent on 1/2 with zero current without rotation. Kinetic limitation of the electrode reaction results in a curvature of the Levich plot ( j vs. 1/2) or, more clearly, according to Eq. (5) in a finite intercept on the abscissa at in the inverse Levich plot ( j − 1 vs. − 1/2), also denoted Koutecky–Levich plot. At conducting polymer film covered electrodes, where different rate and mass transport limited steps have been identified, the situation is more complex, as discussed already by Croissant et al., [23]. In particular, limited diffusion current densities of molecular hydrogen in the bulk electrolyte ( j dif l ) and in the film ( j film ) can be distinguished. l
4.1. Platinum distribution and catalytic acti6ity of Pt/PPy(I) A series of experiments was carried out to deposit platinum onto pre-synthesised PPy films using potentiostatic (Ia), double potential step (Ib), and galvanostatic (Ic) modes of deposition. At first the efficiency of Pt deposition by cathodic reduction of [PtCl6]2 − as function of the applied potential was studied. The amount of deposited Pt was roughly determined by EDX analysis. The intensity of the Pt signal at constant cathodic charge was found to increase when the potential was decreased from 0.3 to − 0.1 V. At more negative potentials (EB − 0.1 V) the EDX signal intensity did not further increase. There are two possible reasons for such behaviour. Depending on the potential applied, platinum is deposited at different depths of the film and is partially not accessible to EDX surface analysis. Alternatively, the deposition process at less cathodic potentials (E\ − 0.1 V) does not proceed with 100% current efficiency due to side reactions, e.g. [PtCl6]2 − reduction to [PtCl4]2 − according to the reaction
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Fig. 1. Cross-section SEM of the Pt/PPy(Ia) film with 0.6 mg cm − 2 Pt load; deposition mode: potentiostatic at E= + 0.2 V.
Fig. 3. Cross-section SEM of the Pt/PPy(Ic) film with 1.0 mg cm − 2 calculated Pt load; deposition mode: galvanostatic with j = −0.1 mA cm − 2.
[PtCl6]2 − +2e− [PtCl4]2 − +2Cl− E 0 =0.726 V vs. SHE
(7)
As shown below, the later explanation seems to be more probable. The efficiency of the Pt deposition at − 0.1 V was considered to be 100%. However, the final preparation of Pt/PPy(Ia) samples using potentiostatic deposition mode (Ia) was carried out at a more positive potential E= +0.2 V. This value ensures both, the necessary cathodic polarisation for the reduction of platinum and sufficient electronic conductivity of PPy. The relatively slow process of Pt deposition at this potential also provides a better condition for the diffusion of [PtCl6]2 − into the PPy film. Under these conditions a current efficiency of 50% was estimated for the Pt deposition process. Samples with
Fig. 2. Cross-section SEM of the Pt/PPy(Ib) film with 0.6 mg cm − 2 calculated Pt load; deposition mode: double potential step with anodic potential E = +0.5 V (10 s); cathodic potential E = −0.1 V (1 s).
different platinum load ranging between 0.13 and 2.9 mg cm − 2 were obtained by varying the deposition time. The SEM micrograph in Fig. 1 shows the cross-section of a Pt/PPy(Ia) sample with a load of 0.6 mg Pt cm − 2. The deposited Pt can be seen on top of the surface. The double potential step method (mode Ib) was used to improve the penetration of the [PtCl6]2 − complex into the PPy film. This was believed to result in a three-dimensional Pt catalyst distribution. The procedure consisted of a periodical sequence of anodic and cathodic potential steps applied to the PPy film in hexachloroplatinate solution. The anodic potential was fixed at 0.5 V. The cathodic potential varied between + 0.2 and −0.5 V. The ratio of the anodic and cathodic pulse period, ~A/~C, was varied from 10 s / 1 s to 5 s / 5 s and the calculated Pt load varied from 0.05 to 2.1 mg cm − 2, respectively. Fig. 2 shows a typical SEM picture of the cross-section of a Pt/PPy(Ib) sample prepared by this method with a calculated Pt load of 0.6 mg cm − 2. Finally, galvanostatic modification of the PPy layer was performed to obtain samples Pt/PPy(Ic). A GC electrode covered with a PPy film was polarised with a cathodic current density of −0.1 mA cm − 2. On varying the polarisation time, different Pt load was obtained ranging between 0.2 and 1.0 mg Pt cm − 2. The SEM picture in Fig. 3 shows a cross-section of such a film with a Pt load of 1.0 mg cm − 2. Because of the relatively small deposition current density the actual electrode potential was not below + 0.2 V, so that under these conditions the parasitic reaction described by Eq. (7) prevailed. This was also indicated by the colour of the hexachloroplatinate solution, which changed from yellowish to orange during the deposition process. Under these conditions a current efficiency of 20% was estimated for the Pt deposition process.
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Fig. 4. Ratio of the Pt to Carbon EDX signal intensities as function of the distance from the film surface; film type and Pt load: O-Pt/PPy(Ia), 0.6 mg cm − 2; 9-Pt/PPy(Ib), 0.6 mg cm − 2,
-Pt/PPy(Ic), 1.0 mg cm − 2; -Pt/PPy(II), 1.8 mM [PtCl6] 2 − in the synthesis solution; D-Pt/PPy(III), 10 mM K2PtCl4 in the synthesis solution.
EDX spectra were taken from the surface and from different locations within the composite film (cross-section). Fig. 4 shows the ratio of the Pt to carbon signal intensity as a function of the distance from the film surface for the different samples prepared. It can be
Fig. 5. Cyclic voltammograms of a Pt/PPy(Ib) sample prepared as descibed in Fig. 2; Pt load 2.1 mg cm − 2, electrode rotation rate 1000 RPM, potential scan rate 20 mV s − 1, electrolyte 0.5 M H2SO4 bubbled with N2. The arrow indicates the change of the peak current with increasing number of cycles.
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clearly seen that the intensity of the Pt:C signal decreases rapidly in the film interior for all modes of Pt deposition on the pre-synthesised PPy film (Ia– Ic). The EDX signal of Pt in the film interior can result either from platinum in the reduced metallic state Pt0 or from the [PtCl6]2 − complex, present as counter ion. There is no direct evidence in our work, but it is more likely that most of the Pt is present in the oxidised form. This was supported by the following experiment. EDX analysis of Pt/PPy(Ib) samples were prepared before and after cyclic voltammetry in 0.5 M H2SO4 under N2 atmosphere. The Pt:C peak ratio of a freshly prepared film measured 0.5 mm inside the film was found to be 0.60 indicating a significant amount of Pt. After cyclic voltammetry the peak ratio decreased to 0.23. The corresponding voltammograms in Fig. 5 exhibit the appearance of a strong cathodic peak with decreasing peak current from cycle to cycle. This may be explained by the reduction of Pt initially present in the form of [PtCl6]2 − , moving from the interior of the film to the film/solution interface, where it is either reduced to Pt0 or dissolved in the bulk of the electrolyte solution. The actual potential of [PtCl6]2 − reduction in solution is approximately 0.5 V [27], whereas the cathodic peak in Fig. 5 appears at more negative potentials around − 0.25 V. This can be explained by a kinetic hindrance of the [PtCl6]2 − reduction in the interior of the PPy film. According to Eq. (1), reduction of the complex would release six chloride ions, which would have to be removed very fast from the film interior to keep local electroneutrality. On the other hand, by reducing the PPy film, negative counter ions (amongst others [PtCl6]2 − ) are transported to the film/ solution interface, where the [PtCl6]2 − complex can be reduced immediately. That is why the Pt reduction peak in Fig. 5 appears directly after the maximum of the PPy reduction peak at 0 V. This all indicates that Pt is predominantly deposited on top of the PPy film. The catalytic properties of the composite films were tested by steady state polarisation curves of the hydrogen oxidation. A set of polarisation curves at different rotation rates is shown in Fig. 6 for electrodes prepared by deposition modes (Ia– Ic) and for a bare platinum electrode Pt. Compared with bare platinum, the polarisation curves of the composite electrodes exhibit higher overvoltage. The rising part of these curves is obviously controlled by the degree of oxidation and the corresponding electronic conductivity of the PPy matrix at low potentials. The values of the transport controlled limiting current densities are higher compared with the bare platinum electrode. The well known decrease of the limiting current density on bare platinum at anodic potentials due to the formation of a PtO layer is less pronounced on samples (Ia) and (Ic) and almost absent on sample (Ib). The reason is probably identical to that of the increased resistance of the Pt/PPy composite
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Fig. 7. Koutecky– Levich plots of the hydrogen oxidation current density at E= +0.2 V on Pt/PPy(Ia) electrodes of different Pt load: O-0.1 mg cm − 2, 9-0.2 mg cm − 2, -0.6 mg cm − 2 and -3.0 mg cm − 2; dashed line represents bare Pt electrode at E= − 0.1 V; electrolyte 0.5 M H2SO4 +H2(1 atm).
potentials, the reaction proceeds under the control of pure mass transfer from the solution to the film surface and presumably by diffusion within the film. Kinetic control can be neglected, except for the samples with low Pt load, where a few Pt islands only cover the surface, as discussed above (partially active electrode).
Fig. 6. Potentiostatic polarisation curves of hydrogen oxidation on Pt/PPy(Ia– Ic) and bare Pt electrodes (RDE) in 0.5 MH2SO4 + H2(1 atm); rotation rate/RPM: O-50, 9-100, 200, -500, D-1000, -2000 and -3000. Pt load: Pt/PPy(Ia) 0.6 mg cm − 2, Pt/PPy(Ib) 0.6 mg cm − 2 and Pt/PPy(Ic) 0.7 mg cm − 2.
against the poisoning of CO [10,11,13,17], i.e. up to now unspecified interactions between conducting polymer and Pt microparticles. Similar sets of polarisation curves were measured on samples Pt/PPy(Ia–Ic) prepared with different platinum loads. From the transport controlled part of these polarisation curves the Koutecky–Levich plots were obtained at a fixed electrode potential E =0.2 V (Figs. 7–9), except for bare Pt with E= −0.1 V. At these
Fig. 8. Koutecky– Levich plots of the hydrogen oxidation current density at E= +0.2 V on Pt/PPy(Ib) electrodes of different Pt load: O-0.2 mg cm − 2, 9-0.4 mg cm − 2, -0.6 mg cm − 2 and -2.1 mg cm − 2; dashed line represents bare Pt electrode at E= − 0.1 V; electrolyte 0.5 M H2SO4 +H2(1 atm).
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Fig. 9. Koutecky– Levich plots of the hydrogen oxidation current density at E = +0.2 V on Pt/PPy(Ic) electrodes of different Pt load: O-0.2 mg cm − 2, 9-0.7 mg cm − 2 and -1.0 mg cm − 2; dashed line represents bare Pt electrode at E= − 0.1 V; electrolyte 0.5 M H2SO4 + H2(1 atm).
Corresponding to this fact are the intercepts of the linear regression lines with the y–axis at . The reciprocal diffusion limited current density, 1/j film , obl tained from the intercept according to Eq. (6), was plotted on a logarithmic scale versus the platinum load, Fig. 10. For the samples (Ia) and (Ib) the decrease of 1/j film with increasing platinum load exhibits two rel gions of different slope. For sample (Ic) this behaviour is not as clear, probably due to the narrow range of Pt loads studied and the limited number of experiments.
Fig. 10. Limiting diffusion current density (1/j film ) obtained l from the intercept of the Koutecky–Levich plots (Figs. 7–9) as function of the platinum load for different sample electrodes: O-Pt/PPy(Ia), 9-Pt/PPy(Ib) and -Pt/PPy(Ic).
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Such behaviour indicates that the process is controlled by two different mechanisms. At Pt loads below approx. 0.5 mg cm − 2 (depending on the Pt deposition mode), the main contribution to the finite intercept originates from the apparent kinetic control of the electrode reaction caused by the low film surface coverage by Pt microparticles. At Pt loads above 0.5 mg cm − 2, the intercept may be referred to the diffusion of hydrogen into the film interior (1/j film ). This increase of l the diffusion rate with the platinum load indicates that the characteristic diffusion length becomes shorter if the density of active platinum sites in the film increases. The data obtained are comparable to those reported by Croissant et. al., [23]. He explains the limitation of the j film at the Pt loads below 0.25 mg cm − 2 by the hydrogen adsorption limiting current. From Figs. 7 – 9 it is also evident that the limiting current densities of the composite electrodes prepared by cathodic deposition of Pt on PPy films are higher compared to the bare Pt electrode and their slope depends on the Pt load. This can be explained by the surface roughness of the PPy film causing more intensive mass transfer between the bulk of the electrolyte and the electrode surface compared to the polished bare Pt electrode. The difference in the slope of the Koutecky– Levich plots originates from the degree of surface coverage of Pt and/or diffusion of hydrogen into the film interior.
4.2. Platinum distribution and catalytic acti6ity of Pt/PPy(II) Samples were prepared by electropolymerisation of the PPy film from colloidal solutions of nano-scale Pt particles. The concentration of the hexachloroplatinate complex added was varied and the amount of Pt incorporated and its effect on the catalytic activity was studied. As demonstrated in Fig. 11 metallic Pt0 particles cannot be observed by SEM. However, the EDX spectra showed a strong and uniform Pt signal over the whole cross section of the PPy film. The ratio of the EDX signal intensity of Pt and carbon is shown in Fig. 4. Furthermore, the relative signal intensity of platinum was found to be proportional to the hexachloroplatinate concentration in the synthesis solution. According to Bose and Rajeshwar [19], the PPy films prepared in this way should contain platinum exclusively in the form of colloidal metallic particles. This statement was supported by XPS surface analysis indicating the presence of Pt0 only. Although not explicitly denoted by the authors, the spectrum was most probably taken from the surface of the sample. By XPS analyses very thin surface layer is probed (approx. 10 nm). However, this layer is strongly influenced by the polarisation procedure during the synthesis or after-
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wards (potential cycling between +0.95 and −0.80 V [19]). Platinum is easily reduced to the metallic form especially at the film surface. In our opinion, inside the PPy film platinum is present in both forms. As already mentioned before, citrates not only reduce [PtCl6]2 − to the form of colloidal Pt0 particles, but also inhibit the charge transfer between the [PtCl6]2 − complex and Py. This means, that to a certain extent [PtCl6]2 − is present in the synthesis solution during electropolymerisation of the PPy film. As shown by Hyodo and Omae [28], large anions are selectively incorporated as counter ions during synthesis of the film. An indication for the presence of [PtCl6]2 − in the film interior was the appearance of a cathodic current peak in the cyclic voltammograms obtained immediately after the electropolymerisation, similar as discussed above in the Pt/ PPy(Ib) system. Moreover, the increase in the Pt signal intensity of the EDX spectra (c.f. Fig. 4) with increasing distance from the film surface indicates that part of the Pt is removed from the interior of the film and is probably replaced by sulphate ions during cyclovoltammetric testing. Potentiostatic polarisation curves of Pt/PPy(II) composite films and the corresponding Koutecky–Levich plots for different Pt loads are shown in Fig. 12. For all Pt concentrations studied, extrapolation leads to near zero intercepts on the 1/j axis; exact values are difficult to obtain because of the slight curvature of the plots. The following data were obtained using linear regression: 0.44 mA − 1 cm2 (with 0.4 mM [PtCl6]2 − in the synthesis solution), 0.28 mA − 1 cm2 (with 1.8 mM [PtCl6]2 − ) and 0.28 mA − 1 cm2 (with 3.0 mM [PtCl6]2 − ). The low values of the intercepts indicate that the reaction proceeds predominantly on the film surface relatively rich on Pt particles.
Fig. 11. Cross-section SEM of the Pt/PPy(II) film synthesised from Pt dispersion solution containing 1.8 mM [PtCl6] 2 − .
Fig. 12. (a) Potentiostatic polarisation curves of hydrogen oxidation on Pt/PPy(II) synthesised from Pt dispersion solution containing 1.8 mM [PtCl6] 2 − ; rotation rate/RPM: O-50, 9-100, -200, -500, D-1000, -2000 and -3000; electrolyte 0.5 M H2SO4 +H2(1 atm). (b) Koutecky– Levich plots of hydrogen oxidation on Pt/PPy(II) synthesised from Pt dispersion solution of different [PtCl6] 2 − concentration: O-0.4 mM, 9-1.8 mM and -3.0 mM; dashed line bare Pt.
The fact that the Koutecky– Levich plots of the two samples prepared from solutions containing 1.8 and 3.0 mM [PtCl6]2 − are almost identical may serve as indirect evidence that the hydrogen oxidation is limited by the amount of Pt on the surface and not in the interior of the film. In the case of the 3.0 mM [PtCl6]2 − solution a Pt EDX signal of similar intensity was recorded from the surface of the film and even a stronger one from inside the film. It can be concluded that, if H2 diffusion occurs into the film interior, it is restricted to a depth of approx. 1 mm (the thickness of the samples studied by EDX).
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4.3. Platinum distribution and catalytic acti6ity of Pt/PPy(III) In the case of potentiostatic polymer film formation at a given potential (E =0.75 V) the anodic current reached a value about one order of magnitude lower than for the previously discussed methods. Since, after prolonged electropolymerisation, PPy formation in the bulk of the solution (darkening) becomes apparent, PPy layers with a thickness of only 3.5 mm were synthesised by this method. Fig. 13 shows an SEM picture of the cross-section of the film synthesised from solution containing 10 mM K2PtCl4. It is not possible to identify individual Pt0 particles distributed in the film by SEM. However, the strong EDX signal of Pt shown in Fig. 4 proves again that Pt is present in the film. It is clear from EDX analysis, that the concentration of Pt in the film interior is several times higher compared to a film synthesised from solution containing colloidal Pt particles. The presence of Pt in the form of crystallites of approximately 10 nm diameter in a PPy film prepared in a similar way was proven by Hepel [13] using transmission electron microscopy. This is apparently in contradiction to the interpretation of the changes observed in the EDX spectra of the Pt/PPy(Ib) samples after cyclic polarisation. However, the situation in both systems is quite different. In the case of the Pt/PPy(III) electrode, the tetrachloroplatinate complex is incorporated during electropolymerisation and according to EQCM experiments of Hepel et al., [13,20] it is fixed in the polymer structure and cannot move out of the film. Moreover, six chloride ions are released during the reduction of the hexachloroplatinate in the case of Pt/PPy(Ib) formation whereas only four are released in the case of Pt/PPy(III). This also causes an important
Fig. 13. Cross-section SEM of the Pt/PPy(III) film electropolymerised from solution containing 10 mM K2PtCl4.
Fig. 14. (a) Potentiostatic polarisation curves of hydrogen oxidation on Pt/PPy(III) synthesised from solution containing 10 mM [PtCl4]2 − , rotation rate/RPM: O-50, 9-100, -200, -500, D-1000, -2000 and -3000; electrolyte 0.5 M H2SO4 +H2(1 atm). (b) Koutecky– Levich plot of the polarisation curves in 14a at E= +0.2 V; dashed line represents the bare Pt electrode.
difference in the preference of the cathodic reduction of the complexes. The polarisation curves of hydrogen oxidation of the Pt/PPy(III) system and the corresponding Koutecky– Levich plots shown in Fig. 14 exhibit a behaviour which differs considerably from that of the previously discussed samples. The polarisation curves (14a) exhibit extremely low overvoltages and the limiting current densities are reached at about E= − 0.2 V remaining constant over the whole potential range explored. The transition of the PPy matrix to the conducting state is not observed. This is in agreement with the crystal microbalance measurements of Hepel [13], according to which ion exchange does not occur and pseudocapaci-
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tance peaks are absent in the characteristics of the potentiodynamic polarisation curve on this type of PPy composite electrodes. According to Hepel, the material does not any more behave like PPy but more like bare Pt. In our case the electrocatalytic activity of the Pt/ PPy(III) composite electrodes was several times lower than that of bare platinum. This is quite surprising for a film of such high Pt content homogeneously distributed in its structure. The high value of the 1/j intercept, 5.80 mA − 1 cm2, of the Koutecky–Levich plots indicate that hydrogen oxidation kinetics must be predominantly controlled by a process different from the mass transfer to the film surface. Further investigations are necessary to understand this phenomenon.
5. Conclusions In the present study it was shown that a real three-dimensional homogeneous distribution of Pt catalyst microparticles inside the conducting polymer film cannot be obtained by cathodic deposition of platinum on a pre-synthesised polypyrrole film. To achieve this, platinum has to be incorporated into the polymer film during electropolymerisation either in the form of colloidal particles or in the form of counter ions, which can be reduced subsequent to electropolymerisation by an appropriate cathodic treatment. The highest Pt loads inside the film were obtained by incorporation of [PtCl4]2 − as counter ion during synthesis of the film and its subsequent reduction. However, such films manifest very low catalytic activity for the hydrogen oxidation reaction in comparison to the films prepared by the alternative methods. Polypyrrole films modified by cathodic platinum deposition on presynthesised polypyrrole film, or platinum incorporation during polymerisation in the form of colloidal particles exhibit electrocatalytic activities comparable to that of bare Pt electrodes. The deposition potential of Pt was found to be an important parameter with respect to the efficiency of the deposition process. There was little evidence of anodic hydrogen oxidation proceeding inside the three-dimensional structure of the composite Pt/PPy catalyst layers.
Acknowledgements The authors are indebted to the Deutsche Forschungsgemeinschaft (DFG) for financial support of this work within the frame of the Schwerpunktprogramm ‘Neuartige Schichtstrukturen fu¨r Brennstoffzellen’ under contract No. JU 201/6. One of us (K.B.) is grateful to the Alexander von Humboldt Foundation for making this co-operation possible.
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